1. Field of the Invention
The present invention relates generally to a hybrid electrically conductive fluid distribution separator plate assembly, a method of making a hybrid electrically conductive fluid distribution separator plate assembly, and systems using a hybrid electrically conductive fluid distribution separator plate assembly according to the present invention.
2. Background Art
Fuel cells are a known power source for many applications including vehicular applications. One such fuel cell is the proton exchange membrane or PEM fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a membrane electrode assembly or MEA. Generally, the MEA is a thin, proton-conductive, polymeric, membrane-electrolyte having an anode electrode face formed on one side thereof and a cathode electrode face formed on the opposite side thereof. One example of a membrane-electrolyte is the type made from ion exchange resins. An exemplary ion exchange resin comprises a perfluoronated sulfonic acid polymer such as NAFION™ available from the E.I. DuPont de Nemeours & Co. The anode and cathode faces, on the other hand, typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive particles such as NAFION™ intermingled with the catalytic and carbon particles; or catalytic particles, without carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder.
Multi-cell PEM fuel cells typically comprise a plurality of the MEAs stacked together in electrical series and separated one from the next by a gas-impermeable, electrically-conductive fluid distribution plate known as a separator plate or a bipolar plate. Such multi-cell fuel cells are known as fuel cell stacks. The separator plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. Electrically conductive fluid distribution plates at the ends of the stack contact only the end cells, and are known as end plates. The separator plates contain a flow field that distributes the gaseous reactants (e.g. H2 and O2/air) over the surfaces of the anode and the cathode. These flow fields generally include a plurality of lands which define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header and an exhaust header located at opposite ends of the flow channels.
A highly porous (i.e. ca. 60% to 80%), electrically-conductive material (e.g. cloth, screen, paper, foam, etc.) known as “diffusion media” is generally interposed between electrically conductive fluid distribution plates and the MEA and serves (1) to distribute gaseous reactant over the entire face of the electrode, between and under the lands of the electrically conductive fluid distribution plate, and (2) collects current from the face of the electrode confronting a groove, and conveys it to the adjacent lands that define that groove. One known such diffusion media comprises a graphite paper having a porosity of about 70% by volume, an uncompressed thickness of about 0.17 mm, and is commercially available from the Toray Company under the name Toray 060. Such diffusion media can also comprise fine mesh, noble metal screen and the like as is known in the art.
In an H2—O2/air PEM fuel cell environment, the electrically conductive fluid distribution plates can typically be in constant contact with mildly acidic solutions (pH 3-5) containing F−, SO4−−, SO3−, HSO4−, CO3−−, and HCO3−, etc. Moreover, the cathode typically operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is typically constantly exposed to hydrogen. Hence, the electrically conductive fluid distribution plates should be resistant to a hostile environment in the fuel cell.
Both metallic and polymeric composite separator plates have been used in the past. Examples of metallic fluid distribution plates can be found in U.S. Pat. Nos. 6,372,376, 6,866,958, and RE37,284 and U.S. Patent Application Publication Nos. 2004/0081881 and 2004/0157108, which (1) are assigned to the assignee of this invention, and (2) are incorporated herein by reference. Examples of polymeric composite fluid distribution plates can be found in U.S. Pat. Nos. 6,607,857; 6,811,918 and 6,827,747 and U.S. Patent Application Publication No. 2004/0062974, which (1) are assigned to the assignee of this invention, and (2) are incorporated herein by reference.
While metallic plates have been found to provide electrically conductive fluid distribution separator plates having acceptable corrosion resistance and contact resistance, metallic plates can be costly and applicants have found certain of them to be more susceptible to corrosion on the anode side of the membrane than composite plates. While polymeric composite plates have been found, to provide electrically conductive fluid distribution separator plates having acceptable corrosion resistance and contact resistance, polymeric composite plates tend to have relatively poor mechanical properties (such as strength) when compared to metallic plates. While both metallic and polymeric composite plates are currently acceptable, there is a desire to provide an electrically, conductive fluid distribution separator plate that overcomes at least one deficiency in the prior art.